1. Field of the Invention
The present invention relates to a semiconductor light emitting device used in an optoelectronic integrated circuit and an image display device, and more particularly to a semiconductor light emitting device and a method for manufacturing the same using a porous silicon.
2. Description of the Related Art
The porous silicon (hereinafter referred to as a "PS") differs from a crystalline silicon (hereinafter referred to as a "c-Si") in optical properties, and absorption edge energy generally becomes large. Moreover, electrical properties also changes, and the resistivity becomes high as compared with the original c-Si. Three kinds of PSs are known as follows:
(a) Nanostructure PS:
The PS of which a porosity is 20 to 80% and the diameter of microporous holes is not more than approximate 2 nm is referred to as "a nanostructure PS". Differing from the c-Si, the nanostructure PS shows luminescence in a visible-light range. Pumping this nanostructure PS by the shorter wavelength light within the spectral region from blue to ultra violet, photoluminescence (PL) of a luminescence efficiency (external quantum efficiency) of approximate 10% at the maximum can be observed. Moreover, electroluminescence (EL) can be obtained also by injecting current into the nanostructure PS.
(b) Mesostructure PS:
On the other hand, the PS of which the porosity is 40 to 60% and the diameter of the microporous holes is approximate 2 to 50 nm is referred to as "a mesostructure PS". The luminescence efficiency of the mesostructure PS is generally low as compared with the nanostructure PS, and an emission wavelength also generally comes to the longer wavelength than the nanostructure PS. The mesostructure PS is coarse in structure as compared with the nanostructure PS and is low in resistivity as compared with the nanostructure PS.
(c) Macrostructure PS:
Moreover, a PS of which the porosity is further low than the mesostructure PS and the diameter of the microporous holes is 50 nm or more is referred to as "a microstructure PS". The macrostructure PS can hardly emit light and is further low in resistivity as compared with the mesostructure PS.
These PSs are formed by anodization, or by feeding a current inwardly from the surface of the silicon through the c-Si (single crystal silicon or polycrystalline silicon) as electrodes in the solution containing hydrogen fluoride (HF). Moreover, as a cathode, materials such as platinum (Pt) being usually not dissolved into an anodization solution is used. Although the PS and the material having the structure similar thereto can be made by other methods, they are omitted because of being not important in the invention.
Thus, the PS is constituted by the number of the microporous holes of the diameter of approximate 1 to 100 nm, remained small c-Si particles or a skeleton, and an amorphous portion surrounding thereabouts. By changing the conditions such as the conductivity type and resistivity of the original c-Si, the current density at anodizing, the composition of the anodization solution, the presence or absence of light irradiation and the intensity of the light irradiation, the structure of the PS being made is changed, whereby the nanostructure PS, the mesostructure PS or the macrostructure PS can be obtained.
For example, the nanostructure PS is obtained by anodizing the c-Si containing a p-type impurity doped to the extent being not degenerated (a non-degenerate p-type). Moreover, the nanostructure PS is obtained also by anodization while irradiating a non-degenerate n-type c-Si of a low impurity concentration with light. This nanostructure PS is fine so that the porosity is approximate 20 to 80% and the diameter of the holes is not more than 2 nm. That is, since remaining c-Si particles or a size of the skeleton are fine, the resistivity becomes high as compared with that of the original c-Si. For example, the nanostructure PS can be obtained by anodizing the degenerate p-type c-Si or the degenerate n-type c-Si, containing the p-type or n-type impurity with higher impurity concentration so that the Fermi level is located within the valence or conduction band. For example, the macrostructure PS can be obtained by anodizing the non-degenerate n-type c-Si in a darkroom.
The above-noted description of the three kinds of PSs which differ in structure is performed on the generalized characteristics of the respectively typical one, and actually, there are the PSs having the characteristic intermediate between the mesostructure PS and the nanostructure PS and the PSs having the characteristic intermediate between the mesostructure PS and the macrostructure PS or the like. Moreover, for example, even the PS belonging to the same nanostructure PS can differ in the fine structure in some cases depending upon the difference of a conductivity type of the original c-Si. Moreover, even though the original c-Si is uniform, the PS of which the structure differs in the direction of a depth can be made depending upon anodizing conditions. Furthermore, even the PS belonging to the mesostructure PS or the macrostructure PS as the general structure and characteristic, the PS containing the nanostructure PS can be made partially in the microscopic portion depending upon the anodizing conditions.
Therefore, when making a light emitting device using the PS, a sufficient consideration should be taken in the both sides of the element design from the viewpoint of by which structure PS a layer is constituted and for what it is used, and a selection of a method for making the element structure.
It is reported in the proceedings of the 44 th Japan Society of Applied Physics and Related Society Symposium, No.2, P.806, Section a-B-6, "Characteristics of a pn-junction type photoanodically fabricated porous silicon LED", by Nishimura, Nagao and Ikeda that external quantum efficiency of the EL luminescence comes to approximate 1% at the maximum in the light emitting device using the PS (hereinafter referred to as a "PS light emitting device"). This PS light emitting device is made by preparing a c-Si wafer that the p.sup.+ type c-Si layer is formed on the n-type c-Si substrate to anodize the surface of this c-Si wafer under the irradiating with light using a lamp. When anodizing under such conditions, the p.sup.+ type c-Si layer of the surface of which resistivity is low becomes the mesostructure PS layer and the n-type c-Si substrate portion of the area which no light from the lamp reaches becomes the macrostructure PS. In FIG. 1 and FIG. 2, the structure and the equipment for manufacturing this PS light emitting device are shown.
Referring to FIG. 1, the macrostructure PS layer 63 made from the n-type c-Si, hereinafter referred to as "a n-type macrostructure PS layer 63", is formed on a n-type c-Si substrate 64. And the nanostructure PS layer 62 made from the n-type c-Si, hereinafter referred to as "a n-type nanostructure PS layer 62" is formed on the n-type macrostructure PS layer 63. And farther the mesostructure PS layer 61 made from the p.sup.+ type c-Si, hereinafter referred to as a p-type mesostructure PS layer, is formed thereon. Moreover, the expressions of "the n-type macrostructure PS layer", "the n-type nanostructure PS layer", "the p-type mesostructure PS layer" or the like are expressed for convenience and differ from the n-type and the p-type in the c-Si. The reason why is that, generally, in the PS layer, acceptor impurities and donor impurities are inactivated at room temperature. A translucent gold electrode 66 which serves as an anode is formed on the p-type mesostructure PS layer 61 and an aluminum electrode 65 which serves as a cathode is formed on the back of the n-type c-Si substrate 64. A direct current power supply 67 for the EL is connected between the anode 66 and the cathode 65. In the structure shown in FIG. 1, the n-type nanostructure PS layer 62 acts as an EL actve layer. Moreover, the p-type mesostructure PS layer 61 has a function to form a junction similar to the pn-junction in the c-Si (hereinafter, such kind of junction by the PS layer is referred to as a "the pn-junction" for convenience) between the n-type nanostructure PS layer 62 and the p-type mesostructure PS layer 61, and a function to get better ohmic contact with the translucent gold electrode 66 formed on the layer 61.
In order to form the structure shown in FIG. 1, first, a c-Si wafer 7 on which a p.sup.+ type c-Si layer 71 of 0.6 .mu.m in thickness and resistivity of 2.times.10.sup.-3 .OMEGA.-cm is formed on a surface of a n-type c-Si substrate 72 of 500 .mu.m and resistivity of 5 .OMEGA.-cm using a thermal diffusion method is prepared. Subsequently, it can be manufactured by anodizing this c-Si wafer 7 as shown in FIG. 2. That is, as shown in FIG. 2, a container 1 for anodization, which is made of polytetrafluoroethylene(PTFE), having an opening on the bottom is contacted closely with the surface of the p.sup.+ type c-Si layer 71 using O-rings 2 to fill an anodizing mixed solution consisting of hydrofluoric acid and ethyl alcohol 4 into this container made of PTFE 1. Because of using the O-ring 2, the anodizing mixed solution consisting of hydrofluoric acid and ethyl alcohol 4 does not leak from the bottom of the container 1 made of PTFE. The anodization solution 4 consists of hydrofluoric acid of 50 weight percent and ethyl alcohol of 99.9 weight percent mixed at a volume ratio of 1:1. In the anodizing mixed solution consisting of hydrofluoric acid and ethyl alcohol 4, a platinum electrode 3 is arranged. On the other hand, on the back of the n-type c-Si substrate 72, the aluminum electrode 65 which will become the cathode shown in FIG. 1 eventually, and a desired anodizing current is fed through the anodizing mixed solution consisting of hydrofluoric acid and ethyl alcohol 4 by the variable direct current power supply 6 connected between the platinum electrode 3 and the aluminum electrode 65. The anodizing is performed while irradiating the p.sup.+ type c-Si layer 71 and the n-type c-Si substrate 72 thereunder by a tungsten lamp 5 arranged on the upper of the container made of PTFE 1. Therefore, the platinum electrode 3 is arranged such that the light radiated from the tungsten lamp 5 can not be impeded to reach the surface of the c-Si wafer 7.
The mesostructure PS layer 61 is obtained by anodizing the p.sup.+ type c-Si layer 71 shown in FIG. 1. Moreover, the nanostructure PS layer 62 shown in FIG. 1 is formed at the portion in proximity to the surface influenced by light radiation in the n-type c-Si substrate 72 shown in FIG. 2. Moreover, the macrostructure PS layer 63 is formed at a slightly inner portion from the surface not influenced by light radiation in the n-type c-Si substrate 72. The n-type c-Si substrate 64 shown in FIG. 1 is a portion remaining as the c-Si of the n-type c-Si substrate 72 shown in FIG. 2. According to the method shown in FIG. 2, the longer an anodization time is, the thicker the nanostructure PS layer 62 becomes, and moreover, the macrostructure PS layer 63 formed thereunder comes to be thick increasingly in response thereto. Moreover, the anode 66 shown in FIG. 1 is the translucent gold electrode formed by evaporating a gold thin film by a vacuum evaporation method after anodizing.
The luminescence efficiency (the quantum efficiency) depends upon a way of anodizing and the anodization time, thereby not always being constant. Generally, the nanostructure PS layer anodized sufficiently by extending the anodization time has higher luminescence efficiency than that of the nanostructure PS layer anodized insufficiently with the shorter anodization time.
To some extent, the longer the anodization time becomes, the higher the external quantum efficiency and higher the electric power efficiency of the PS light emitting device become. The reason why is that when the anodization time is made long, so that anodizing is promoted sufficiently, the quantum efficiency increases. However, when the anodization time is long excessively, the electric power efficiency decreases again. This reason why is that, although the external quantum efficiency becomes higher owing to the increase of the thickness of the nanostructure PS layer 62 being the light emitting layer in company with the increase of the anodization time, the increase of the series resistance of the nanostructure PS layer whose resistivity is high becomes prominent when exceeding a certain thickness. This is to be understood by referring to FIG. 3. That is, FIG. 3 shows a relationship between a series resistance Rs of such PS light emitting device and a thickness "d" of the nanostructure PS layer 62. Referring to FIG. 3, a symbol of O shows the series resistance Rs of the light emitting device having the n-type nanostructure PS layer 62 shown in FIG. 1. And a symbol of .diamond. shows the series resistance Rs of the light emitting device, in which a luminescence layer is constituted by the p-type nanostructure PS layer made from the p-type c-Si, having the approximately same element structure as that shown by the symbol of O. It is understood that there is a relationship of approximately Rsd.sup.2.about.3. Therefore, for the light emitting device whose external quantum efficiency is high, the series resistance thereof becomes high inevitably. Especially, the series resistance Rs of the PS light emitting device, whose external quantum efficiency is high as 0.1 to 1%, becomes high as 100 k .OMEGA. to 1M .OMEGA., and a high supply voltage is required in order to inject a current into such PS light emitting device. That is, electric energy converted to thermal energy is more increased with respect to electric energy converted to light energy, whereby the electric power efficiency would be decreased.